The present invention relates to optical sights, in particular to telescopic optical sights equipped with a switchable image intensifier tube, which can be switched between the non-operative position for use of the telescopic optical sight alone or to an operative position in which the tube can be used in combination with the telescopic sight for enhancing operation of the latter.
There exist a great variety of optical sights of different types, in particular for application on hunting, combat, or training weapons. In general a sight is utilized for aiming a weapon during daytime operation at a directly visible target and during nighttime operation at a target, which is only visible through the use of some vision-aiding devices.
Daytime sight devices range from simple front and rear sights like those on ordinary rifles to complex optical systems in combination with laser range finders and laser aiming devices. Certain sophisticated types such as telescopes are utilized to magnify a target and to enable the user during normal daytime operation to view a magnification of the target area at which the viewer is aiming.
There are also nighttime vision devices or sights, which employ image intensifiers or similar structures. The function of an image intensifier is to multiply the amount of incident light received by it to produce a signal that is bright enough for presentation to the eyes of a viewer. As such, these devices have been employed by the military and in commercial products as well. Sights vary in size, magnification, type of reticle, weapon application and level of performance.
An image intensifier tube (IIT) is a vacuum photoelectronic device intended either for transformation of an invisible IR, UV, or X-ray image of an object into a visible image or for intensification of a visible image. An IIT normally consists of a photocathode, an image intensification system, and a cathode-luminescent screen. The photocathode transforms the original optical image into a so-called electronic image. With the use of the image-intensifying system, the electronic image is transferred to the screen where this image, in turn, is converted into a visible original image. In the IIT, the light reflected from the object causes emission of electrons (photocurrent) from the surface of the photocathode. In this case, a magnitude of photocurrent generated by various areas of the photocathode depends on distribution of density of images projected onto these areas. Photoelectrons accelerated and focused by the IIT""s field, bombard the screen, thus causing it to luminesce. Since brightness on individual areas of the screen depends on density of the photocurrent, the screen reproduces a visible image of the object.
In its simplest form an IIT consists of two parallel electrodes, i.e., a photocathode and a screen, between which a voltage is applied. In a uniform electrostatic field of such an IIT, electrons are practically not focused (the electrons move along parabolas having parameters dependent on initial velocities of the electrons). For focusing of electrons, the IIT with a uniform electrostatic field is placed into a uniform magnetic field having the same direction as the electric field. In this case, the electrons emitted from individual points of the cathode begin to move along periodically converging spiral paths rather than along the diverging parabolas. The use of immersion-type electrostatic lenses makes it possible to obtain a good electronic image, even without the use of a magnetic field.
In an IIT, intensification of the original image is achieved due to additional acceleration imparted to the electrons as well as due to compression of the electronic image. In this case, brightness is also increased with a factor of 1/B2, where B is an optolectronic magnification. Brightness is increased with the use of a multiple-stage IIT, which comprises several IITs connected in series. From the screen of the first IIT, a luminous flow is directed to a photocathode of the second IIT, etc. Normally, multiple-stage IITs are encapsulated into a common shell. In order to prevent significant loss in resolution capacity, a thickness of a transparent partition between the stages should not exceed 5 to 10 xcexcm.
Application of optical fiber plates makes it possible to connect individual IITs via direct optical contact between the surfaces of the plates. Multiple-stage IITs provide the maximum possible amplification of brightness when the output cathode-luminescent screen reproduces elements separately emitted from the photocathode. An IIT with a microchannel plate provides intensification of brightness close to the maximum possible limit. A microchannel plate is a glass plate with several million channels (having diameters within the range of 5 xcexcm to 15 xcexcm) with a voltage of about 1 kV applied to the end faces of this plate. In such an IIT, the electronic image is aligned with an input surface of the microchannel plate and is divided by the channels into separate elements. On its way through the channels, the electron flow of each element is multiplied by 103 to 104 times due to secondary emission of electrons caused by collision of the electrons with the walls of the channels. The obtained electronic image of increased density is transferred to the screen.
A basic parameter of an IIT is an integral sensitivity, which is a ratio of the photocurrent to a value of a light flow incident on the photocathode. For example, in an IIT with an oxygen-silver-cesium cathode intended for conversion of images in infra-red rays with the wavelength of 1.3 xcexcm, image sensitivity may reach 50 mkA/lumen. A multiple-alkaline photocathode which contains compounds of Sb with Cs, K, and Na and which is used in IIT for amplification of a visible image, provides integral sensitivity up to 400 mkA/lumen. Other basic parameters are a resolution capacity (which is determined by the amount of separately seen black-and-white lines or dots per unit of length and which is within the range of 25 to 60 mmxe2x88x921, or higher); a coefficient of transformation (a ratio of the luminous flow emitted from the screen to the luminous flow incident on the photocathode and which reaches several hundred in single-stage IITs and 5xc3x97104in multiple-stage IITs; and time resolution, which in latest IITs reaches 10xe2x88x9212 sec. Among other applications, the parameters listed above make it possible to use IITs also in night-vision systems, such as optical arm sights, as well as in range finders utilizing back-light systems for pulse illumination of objects, where illumination pulse may have time resolution of up to 10xe2x88x9212 sec.
In general, an IIT alone is unsuitable for use as an optical sight because search of remote targets requires optical magnification and superposition of the target image onto the photocathode of the IIT. The above function is fulfilled by an objective lens, which isolates the area of interest and magnifies the target found in this area. Matching of the image reproduced by IIT with the pupil of the viewer""s eye requires the use of an eyepiece. Thus, the IIT used in the optical sight comprises a complex optoelectronic system, which consists of an IIT per se, an objective lens unit, an eyepiece lens unit, and electronics.
Since it is advantageous to use the same weapon with the daytime and nighttime vision devices, many contemporary weapons are provided with possibility of installing both a daytime or nighttime sights.
For example, U.S. Pat. No. 4,822,994 discloses a configuration in which the front end of a telescopic sight is separable from the rest of the sight. For nighttime use an image intensifier module is inserted between the sections. However, for daytime operation, the user must disassemble the sight and remove and store the image intensifier module.
U.S. Pat. No. 4,629,295 is directed to another type of night viewing device, which is an add-on accessory to day rifle sights. This device is attached directly to the day rifle scope. It includes an objective assembly, which receives the image and directs it to a night vision device. The intensified image is then directed to the day sight for viewing. Again, the device is bulky and must be carried by the user or stored during daytime operations.
One disadvantage to having separate daytime and nighttime sights is that the sights must be individually boresighted to the weapon whenever the sight is initially installed, and must be checked for boresight whenever the sight is reinstalled on the weapon. Current use of weapon sights by law enforcement and military personnel and by civilian users involves the careful mounting and boresighting of a day and/or night vision sight to the weapon. For maximum accuracy, actual firing of the weapon is required during the boresighting process. This is not generally feasible under combat conditions. Separate weapon sights are also disadvantageous because the sights must be interchanged for day or night use. In addition, the separate night vision sight adds an additional three to four pounds, which must be carried and handled separately by the user.
On the other hand, it is understood that the lenses of the objective lens unit and of the ocular lens unit may form a conventional optical daytime sight, which comprises a conventional telescopic tube with adjustable magnification. It is obvious that such daytime conventional telescopic sight can be combined with a night vision device such as IIT to form a self-contained device, and a great variety of optical sight systems which can at the same time be used both in daytime and nighttime merely by switching and without disconnection have recently appeared on the market and became a subject of new patents.
For example, U.S. Pat. No. 5,084,780 issued in 1992 to E. Phillips and U.S. Pat. No. 5,946,132 issued to the same inventor in 1999 disclose a telescopic sight which can be used for either nighttime or daytime operation and is particularly adaptable for use on weapons ranging from rifles to anti-tank weapons. A first embodiment includes a single objective and two parallel light paths, one for day viewing and one for night viewing. The objective forms the beginning of the night path. Separating dichroic mirrors transmit light from the objective along the night path and reflects light from the objective to the day path. The night path includes an IIT. A mirror at the end of the night path reflects the light from the IIT to a beamsplitter/combiner on the day path. The beamsplitter/combiner transmits the light from the day path and reflects the light from the night path along the same path to an ocular assembly for viewing. A second embodiment of the telescopic sight is similar to the first embodiment but contains two objective lens assemblies for collecting light, one for the night path and one for the day path. Because the sight has two separate objective lens assemblies, separating mirrors are not included. A third embodiment includes a projected aiming reticle and a direct view capability for day viewing which replaces one of the objective lens assemblies. The direct view channel includes a beamsplitter/combiner.
Furthermore, both patents of Phillips describe various methods for introducing an image of a reticle to the combined day and night optical path.
In spite of all advantages of the aforementioned optical sight system, it utilizes two parallel optical paths always used simultaneously irrespective of nighttime or daytime application. It is obvious that the use of two separate optical paths at the same time makes the sight optics large and heavy, which is a significant drawback for weapons, which are manually carried by the user.
The device based on the same principle of simultaneous and constant use of daytime and nighttime optical paths is disclosed in another U.S. Pat. No. 5,902,996 issued in 1999 to K. Sauter. This device is provided with two rotating mirrors, which can be rotated simultaneously for opening or closing the night vision system. Although this system is more reliable than the previous one, in general it entails the same disadvantages since it does not suggest any other new solutions of the problems inherent in the sight with two parallel and simultaneously working optical paths.
The above problem is solved by the system described in U.S. Pat. No. 6,131,294 issued in 2000 to U. Jibiki. This device comprises a telescopic sight with a small separate night-vision block insertable into the daytime optical path between the ocular lens and the objective lens. A special recess is formed in the sight housing for fitting the insertable block into this recess with alignment of the optical path of the night-vision insert with that of the daytime sight portion. Insertion is carried out without the use of any special instruments or fasteners.
Although the U.S. Pat. No. 6,131,294 solves the problems of the earlier described devices by providing a single-path day/night vision optics, it is still possesses a number of significant disadvantages, which are the following.
Disconnection of the night-vision device, such as image intensifier, exposes two optical surfaces on the opposite sides of the recess. During the use of the sight in the daytime mode these optical surfaces remain unprotected. Penetration of scattered day light into the aforementioned recess contributes to decrease in contrast of the image. Furthermore, the night-vision device, such as an image intensifier tube, is a very delicate optical instrument, which requires accurate handling after replacement. Therefore the use of the device of U.S. Pat. No. 6,131,294 is unsuitable for combat field conditions.
When the night vision insert has a temperature different from the temperature of the stationary part of the sight, insertion of the night-vision block may cause fogging of the sight optics, which can make the sight inoperative for a substantial period of time.
Another problem associated with the use of the insertable night-vision block consists in that each insertion and removal requires readjustment of the optical system for refocusing.
U.S. Pat. No. 4,961,278 issued in 1990 to Ch. Johnson, et al., describes a sight apparatus for selective daytime and nighttime use due to a provision of a rotating housing located between the ocular and objective of the telescopic sight assembly. This housing contains an image intensifier unit for night time use and lens coupling assembly for daytime use which are rigidly attached to opposite arms of a two-arm lever so that rotation of this level by about 180xc2x0 will alternatingly align the optical axis of the sight either with the image intensifier unit or with the daytime lens coupling assembly which is placed in the optical path of the sight. A disadvantage of this device consists in that in addition to an image intensifier unit it requires the use of an auxiliary daytime coupling assembly, which both have to be attached to opposite arms of a pivotable lever. The use of the auxiliary daytime coupling contributes to an increase in the weight of the sight. Furthermore, as the coupling assembly and the image intensifier have diametrically opposite positions and the lever that supports these units has to be rotated by 180xc2x0, in order to have enough room for such rotation the sight should have increased overall dimensions. Moreover, switching between the daytime optics and nighttime optics requires manual focusing and magnification with the use of a focusing and magnification mechanisms. Another disadvantage of the design described in the aforementioned patent is that switching between the daytime use and the nighttime use requires rotation of the aforementioned lever with the entire housing and with switchable optics respective to the sight housing. This means that the housing of the sight consists of two separate parts, one of which is rotatingly installed on the other. Another essential disadvantage of the sight disclosed by he aforementioned patent consists in that arrangement of rotatable optics for rotation in a plane perpendicular to the optical axis of the sight. Such an arrangement contributes to an additional significant increase in the sight dimensions because for efficient operation the image intensifier requires a high aperture ratio and this, in turn, requires the use of electropotics of large diameter. This leads to an increase in a vertical dimension of the sight.
It is an object of the present invention to provide an integral day/night optical sight, which is simple in construction, small in size, light in weight, utilizes a single optical path with the minimal number of lenses, does not change the temperature of the sight when the night-vision block is installed into the optical path of the daytime optics, does not require disconnection of the night-vision unit from the sight housing, provides reliable operation in day and night modes, and automatically readjusts sections of the optical paths when the night-vision unit is switched between the daytime and nighttime operation positions. Another object is to provide a self-contained day/night sight in which switchable nighttime optics and daytime optics are located in a single housing. Still another object is to provide the sight of the aforementioned type, which does not require the use of any optical compensation units for switching between the daytime and nighttime use.
The self-contained day/night optical sight device of the invention has a sealed sight housing permanently attached to the weapon or to other object and contains an objective lens and an eyepiece lens installed on a common optical path at a distance from each other so that a space is formed between the both. The same sealed housing pivotally supports a night-vision unit, such as an image-intensifier tube, which can be turned in the plane that contains the optical axis of the sight between the position offset from the aforementioned common optical axis and the position coincident with this optical axis. Since both the night-vision and day-vision optics are located in a sealed housing, the lenses are protected from contamination and fogging. The use of a single optical path makes it possible to reduce the weight of the system. Rotation of the night-vision unit to the working position is interlocked with the day-vision optics so that switching of the sight to night-vision conditions will automatically shift the daytime optics back for a distance required to match both optics.